1. Field of the Invention
The present invention relates to a semiconductor laser gyro and its driving method and, in particular, to a method for driving the semiconductor laser gyro for generating a laser oscillation by injecting a current.
2. Related Background Art
In the past, there have been known mechanical gyros having rotators and vibrators or optical gyros for detecting the angular velocity of a moving object. Since the optical gyros, in particular, are capable of starting instantaneously and have wide dynamic ranges, innovation is being brought about in the field of gyro technology. The optical gyros include laser gyros, optical fiber gyros, passive resonator gyros and the like. Among them, a development of a laser gyro employing a gas laser was first undertaken and it has been already put to practical use in aircrafts and the like. Recently, as a small and highly accurate laser gyro, a semiconductor laser gyro integrated on a semiconductor substrate has been proposed, for which known literature is available in the form of Japanese Patent Application Laid-Open No. 5-288556.
In order to increase a detecting sensitivity for the angular velocity, when a length of a ring cavity of a ring laser is taken as L and a closed area surrounded by the optical path as S, S/L is made large, by which a beat frequency accompanied by a rotation may be increased. However, there was a problem in that a driving current becomes large if S/L is made large in the semiconductor laser gyro. This is because the start of a laser oscillation is determined by a carrier concentration in an active layer, and the driving current is increased in proportion to the area of the active layer.
An object of the present invention is to provide a semiconductor laser gyro in which the time average value of the driving current is small, and a method for driving the semiconductor laser gyro.
According to an aspect of the present invention, there is provided a ring laser gyro having means for injecting a current of a first current value not less than an oscillation threshold value to a ring laser gyro, and then injecting a current of a second current value smaller than the first current value to the ring laser gyro. In the ring laser gyro, a beat signal can be detected even in a state wherein the current of the second current value is injected and flowing. In the ring laser gyro, circularly counterpropagating first and second laser beams may be different from each other in the oscillation frequency when the gyro is stationary. Further, the second current value may be lower than the oscillation threshold value. Also, in this embodiment of the ring laser gyro, a beat signal can be detected even in a state wherein the current of the second current value is injected and flowing.
According to another aspect of the present invention, there is provided a method for driving a ring laser gyro, which involves injecting a current of a first current value not less than an oscillation threshold value to a semiconductor laser gyro, and then injecting a current of a second current value smaller than the first current value.
According to still another aspect of the present invention, there is provided a method for driving a ring laser gyro, which involves injecting a current of a first current value not less than an oscillation threshold value to a semiconductor laser gyro, and then injecting a current of a second current value smaller than the first current value, which is taken as one pair of operations, and repeating the pair of operations plural times. The method for driving a ring laser gyro may further comprise injecting a current of the first current value in the next pair of the operations while the laser beams are propagating inside the ring resonator after the injecting step of the current of the second current value, wherein the semiconductor laser gyro starts a laser oscillation by the injected current, and propagates laser beams inside the ring resonator.
In the above methods for driving a ring laser gyro, the current of the second current value may be not less than a current value where a medium becomes transparent.
In the above method for driving a ring laser gyro, the current of the second current value may be not less than the oscillation threshold current.
In the above method for driving a ring laser gyro, a sidewall of the semiconductor laser gyro is a surface with total reflection for oscillated laser beams.
The operation of the above configuration will be described by using an equation. First, the beat frequency xcex94f in the laser gyro can be expressed as follows by using a cavity length L of the ring laser, a closed area S surrounded by the optical path, an oscillation wavelength xcex and an angular velocity xcexa9:                               Δ          ⁢                      xe2x80x83                    ⁢          f                =                                            4              ⁢              S                                      λ              ⁢                              xe2x80x83                            ⁢              L                                ⁢                      xe2x80x83                    ⁢          Ω                                    (        1        )            
This equation demonstrates that S/L should be increased in order to make the beat frequency xcex94f large. However, if S/L is made large in the semiconductor laser gyro, a driving current becomes large. This is because the start of the laser oscillation is determined by the carrier concentration in the active layer. Consequently, the driving current is increased in proportion to the area of the active layer. In relation to this, the present invention proposes a driving method in which a time average value of the driving current is small and its structure.
Rate equations concerning a photon density S of the laser beam, a photon density SSP of a spontaneous emission and the carrier concentration n can be given by the following equations:                                           ⅆ            S                                ⅆ            t                          =                              G            ⁢                          xe2x80x83                        ⁢                          (              n              )                        ⁢                          xe2x80x83                        ⁢            S                    +                                    β              s                        ⁢                          xe2x80x83                        ⁢                          n                              τ                r                                              -                      S                          τ              ph                                                          (        2        )                                                      ⅆ                          S              sp                                            ⅆ            t                          =                              n                          τ              r                                -                                    S              sp                                      τ              ph              xe2x80x2                                                          (        3        )                                                      ⅆ            n                                ⅆ            t                          =                              I            eV                    -                      G            ⁢                          xe2x80x83                        ⁢                          (              n              )                        ⁢                          xe2x80x83                        ⁢            S                    -                      n                          τ              n                                +                                    G              xe2x80x2                        ⁢                          xe2x80x83                        ⁢                          (              n              )                        ⁢                          xe2x80x83                        ⁢                          S              sp                                                          (        4        )            
wherein G(n) denotes a gain coefficient for the laser beam, xcex2s a spontaneous emission coupling factor, xcfx84r a radiative recombination lifetime of the carrier, xcfx84ph a photon lifetime for the laser beam, xcfx84phxe2x80x2 a photon lifetime for the spontaneous emission, I an injection current, e an elementary electric charge, V a volume of the active layer, xcfx84n a carrier lifetime, and G (n)xe2x80x2 an absorption coefficient for the spontaneous emission.
From the above, a steady state d/dt=0 is considered. Since the spontaneous emission coupling factor xcex2s is in the order of 10xe2x88x924 to 10xe2x88x926, if the contribution of xcex2s is ignored in equation (2), the following relation holds:                               G          ⁢                      xe2x80x83                    ⁢                      (                          n              th                        )                          =                  1                      τ            ph                                              (        5        )            
Also, from equation (4), the oscillation threshold current Ith becomes as follows:                               I          th                =                              eV                          τ              n                                ⁢                      xe2x80x83                    ⁢                                    n              th                        ⁡                          [                              1                -                                                      G                    xe2x80x2                                    ⁢                                      xe2x80x83                                    ⁢                                      (                                          n                      th                                        )                                    ⁢                                      xe2x80x83                                    ⁢                                      τ                    ph                    xe2x80x2                                                              ]                                                          (        6        )            
Here, nth is a carrier concentration of the oscillation threshold, and if the contribution of a nonradiative recombination is negligible, the following equation:
xcfx84r≅xcfx84n
is established.
Moreover, from equation (2) and equation (4), the photon density S in Ixe2x89xa7Ith is given by the following equation:                     S        =                              τ            ph                    ⁡                      [                                          I                eV                            -                                                n                  th                                                  τ                  n                                            +                                                G                  xe2x80x2                                ⁢                                  xe2x80x83                                ⁢                                  (                  n                  )                                ⁢                                  xe2x80x83                                ⁢                                                      τ                    ph                    xe2x80x2                                                        τ                    r                                                  ⁢                                  xe2x80x83                                ⁢                                  n                  th                                                      ]                                              (        7        )            
Now, when injection current I is once taken as a value I1 not less than the oscillation threshold current Ith, the laser oscillation begins. If it is given an in-plane rotation including a ring resonator in this state, the circularly counterpropagating laser beams interfere with each other to generate a beat. By detecting this beat frequency, the angular speed of the rotation is known.
Next, after the laser oscillation, injection current I is lowered to I2 ( less than I1). The laser beam circularly propagates inside the ring resonator after the laser is oscillated. Particularly, if the sidewall of the element is a total reflective surface, the laser beam is not emitted outside the ring resonator but remains inside the resonator all the time. That is, the photon lifetime xcfx84ph for the laser beam becomes long, and even if injection current I is lowered, the laser beam can be kept propagating inside the ring resonator. As a result, the time averaged value of the current injected to the element can be lowered, and a load of the current source can be lowered which, at the same time, contributes to the lowering of a consumption power.
The injection current I2 may be smaller than the oscillation threshold current Ith. In this case, however, the laser oscillation is discontinued as time elapses and a signal ends up being not outputted. Nevertheless, by making injection current I2 larger than injection current I0 where a medium becomes transparent, an amplification by stimulated emission takes place. For this reason, the time where the laser beam exists inside an optical resonator can be prolonged. Moreover, if injection current I2 is not less than the oscillation threshold current Ith (I2xe2x89xa7Ith), the laser oscillation continues once the laser is oscillated.
In order to lower the consumption power while the laser oscillation is continued all the time, the following method is effective. That is, an operation is repeated, which operation consists of restoring the injection current I to a value I1 not less than the oscillation threshold current Ith during the time when the laser beams propagate inside the ring resonator and then making injection current I lowered to I2 ( less than I1). In this manner, the laser beams of an approximately constant optical intensity can be kept existing inside the optical resonator. As a result, the beat signal of the same magnitude can be obtained all the time. Needless to say, the consumption power in this method is small in comparison to that at the time when the constant current I1 is continuously injected to the laser gyro. It will be sufficient if the interval between current I1 and current I2 is that enabling beat signals to be consecutively obtained.
And, if the sidewall of the element is a total reflective surface, the photon lifetime xcfx84phxe2x80x2 for the spontaneous emission also becomes long. In the case of the semiconductor laser, while a gain is increased with the injection current, a population inversion is not produced for an optical transition with an energy much higher than a gain peak, and absorption takes place. In the spontaneous emission, a photon whose transition energy is inside the absorption region is absorbed, during which it is kept remaining inside the ring resonator and reproduces the carrier. This is photon recycling and has an effect of prolonging the carrier lifetime. As a result, when injection current I is made small, the time when the laser beam remains inside the ring resonator becomes long. Moreover, similar to a function of a second term of equation (6), the lowering of the oscillation threshold current Ith is achieved.
Needless to say, even if the sidewall of the element is not a total reflective surface, if its reflectivity is great, the same effect can be expected. However, the greater the reflectivity of the sidewall is, the smaller the oscillation threshold concentration nth becomes and a slope effect for the laser beam inside the optical resonator becomes great simultaneously. The effect of the photon recycling also becomes efficient. Note that, in the ring laser having a ridge waveguide, a loss difference arises in the counterpropagating laser beams if non-symmetric tapered areas are provided as shown in FIG. 8. Due to the loss difference, the oscillation frequencies of the counterpropagating laser beams are different from each other. In this case, the beat signal (f0) can be obtained even when the gyro is stationary. By comparing f0 with the signal, a rotating direction of the gyro can also be detected. However, also with the aim of avoiding a lock-in phenomenon, it is desirable to provide the tapered areas so as to let the difference in oscillation frequency between the counterpropagating laser beams amount to not less than 100 Hz, desirably not less than 1 kHz, or more desirably not less than 10 kHz. Further, if the difference in optical intensity between the counterpropagating laser beams is too large, a degree of interference of the beams may be low, whereby no beat may be generated. Accordingly, it is desirable to provide the tapered areas so as to let the lower optical intensity amount to not less than 30%, desirably not less than 50%, or more desirably not less than 80%, of the higher one.